Workflow for modeling assets like this (picture included)
There are many ways, it depends of the type of asset you want to create (asset with handpainted texture, etc.).
 I use this sometimes:


model low poly first
UV unwrap the low poly model
texturing of the low poly model.  In "properties" panel, go to "data" tab and then "UVmaps", add a second UV map by clicking on the plus button, highlight this second by clicking on it, unwrap the low poly model for a second time for a good unwrapping because of the texturing
create a new image, bake textures and normal maps and remove all the materials and the the second UVmap
duplicate the low poly, add more details to the second low poly models and  bake normal map again.
And you can export textures if you want for more details on Photoshop or Gimp.